Gas (combustion) turbine engines generate power for a variety of applications including land-based electrical power generating plants. The turbines are designed to combust a broad range of hydrocarbon fuels, such as natural gas, kerosene, biomass gas, liquid natural gas, synthetic gas, etc. Gas turbines produce an exhaust stream containing a number of combustion byproducts, many of which are considered atmospheric pollutants. Increasingly stringent regulations have been imposed on the operation of gas turbine power plants in an effort to minimize the production of these gasses. Regulating production of the various forms of nitrogen oxides, collectively known as NOx, is of particular concern.
It is known that gas turbine NOx emissions increase significantly as the combustion temperature rises. One method of limiting the NOx production employs the lean premixed concept where the fuel and combustion air are mixed prior to reaching the combustion zone at a relatively low fuel-to-air ratio. Limiting the peak combustion temperature reduces the NOx emissions. In this design the degree of mixing between the fuel and air is critical to the performance of the combustion system.
Typically, a gas turbine engine comprises one or more injectors for injecting fuel into air (i.e., primary zone air) upstream of a combustor where the fuel burns. The fuel injectors of conventional turbine engines may be arranged in one of at least three different schemes. In a lean premix flame system, the fuel injectors are positioned to inject fuel into the air stream at an upstream location that is sufficiently separated from the flame zone to allow complete mixing of the fuel/air mixture prior to reaching and burning in the flame zone. Fuel injectors are configured in a diffusion flame system to simultaneously mix and burn the fuel and air. In a partially premixed system, the fuel injectors inject fuel upstream of the flame zone a sufficient distance to allow some of the air to mix with the fuel prior to reaching the flame zone. A partially premixed system is a combination of a lean premix flame system and a diffusion flame system.
To avoid local hot spots that produce a high level of NOx emissions, a low-emission gas turbine combustion engine requires thorough mixing of the fuel and air streams prior to reaching the combustion zone. Preferred techniques for mixing the air and fuel streams are dependent on a momentum (mass multiplied by velocity) ratio of the two flow streams. With the current emphasis on alternative gas turbine fuels, it is desired that the gas turbine components function properly with different fuels. Fuel/air mixing apparatuses that rely on the momentum ratio to mix the fuels are satisfactory for only a narrow range of momentum ratios and therefore are limited to certain fuels. For a gas turbine combustor that can perform efficiently using a wide range of fuels, it is necessary to develop a method for fuel air mixing that is independent of the momentum ratio of the gas streams.
One known prior art technique for mixing air and fuel, as illustrated in FIG. 1, comprises a fuel manifold 10 bounding an air stream represented by arrowheads 18. One or more fuel injectors (not shown) inject fuel into the air stream cross flow 18 through a plurality of openings 20 in a surface 10A of the fuel manifold 10, forming a fuel mixing region indicated generally by a reference character 24. Disadvantageously, the extent to which the fuel and air mix depends on the penetration of the injected fuel into the air stream 18, which in turn is determined by the ratio of the momentum of the two streams.
For optimal mixing the fuel should penetrate about ⅔ of the air stream 18. Excessive fuel penetration causes the fuel to strike an interior lower wall surface 10B of the manifold 10, creating a recirculation zone 30 as illustrated in FIG. 2. Poor mixing within the recirculation zone 30 can lead to flashback in the manifold 10.
If the fuel does not penetrate a sufficient distance into the air stream 18, the air/fuel mixture is stratified with a majority of the fuel proximate the upper surface 10A. In this case, the poorly mixed fuel results in a degradation of performance and increased NOx emissions.
The momentum ratio of the fuel and air streams varies according to fuel type, fuel heating value and fuel density. To ensure proper air-fuel mixing, such as when the fuel supplied to the combustion turbine is changed, it is necessary to adjust the size and/or location of the fuel injection openings 20. For a combustion system that operates with many different types of fuel, it may be necessary to utilize more than one fuel manifold, each manifold having differently sized and/or located injection openings for use with a specific fuel type. As is known, current gas turbines must be capable of burning a wide variety of fuels including natural gas, liquid natural gas, syngas and hydrogen. Designing a gas turbine with multiple manifolds, each designed specifically for one fuel type is not a practical solution. Neither is it desired to remove and replace the fuel manifold whenever a different fuel type is to be burned.
The problem of rapidly and thoroughly mixing the fuel and primary zone air has been addressed by the use of swirlers. Axial swirlers are disposed within the fuel injector or external the fuel injector along the fuel flow path to swirl the fuel flow and improve the air-fuel mixing process. A plurality of closely-spaced swirl cups downstream of the fuel injection point can enhance the air-fuel mixing process. Although these prior art techniques have helped to reduce NOx emissions, combustion performance can be further improved by further increasing the efficiency of mixing fuel and primary zone air prior to fuel combustion.
Therefore, there is a need for an air-fuel mixer to further improve the process of mixing fuel and primary zone air. There is a further need for an efficient air-fuel mixer for use with various types of fuels, independent of the fuel momentum as it enters the air flow.